Airborne sonar systems include a hoist mechanism carried in a helicopter, a storage drum and drive means therefor which operate to lower a sonar transducer into the water and raise it out of the water. When raised, the transducer is carried snugly against a conical seat which prevents it from swinging or moving during flight. Older types of airborne sonar use comparatively heavy transducers with large, thick cables having many strands and are limited in the depth to which the transducer is operated. Although the hoist mechanism is operated at limited speeds, there are problems connected with various aspects of the retrieval. The force required to pull the transducer through the water is greater than that required in air; hence, there is a danger that the transducer may tend to jump out of the water and swing in an uncontrolled manner for a time, which complicates operation of the helicopter. Further, means must be provided to insure that the transducer does not hit against the seat with excessive force. A further problem is that the cable tends to carry a substantial amount of sea water with it as it is reeled in. Since the hoist is within the helicopter and since the water tends to be sprayed all around as the cable passes over the guide sheaves, it has been found necessary to provide a cable wiper between the seat and the guide sheaves to prevent excessive amounts of water from being carried on board the helicopter.
With the advent of later types of airborne sonar, transducer operating depths have become much greater, and cable speeds upon deployment and retrieval have also become much greater. While the cables themselves are smaller in diameter, the greater speed to which they are subjected results in their carrying even greater quantities of water, at least on a per-unit-of-time basis. With the greater reeling speed and the quantity of water carried it was found that, in the absence of a cable wiper, the spray of sea water as the cable made contact with the guide sheave was intolerably heavy. It soon became apparent that the type of wiper used on the earlier, larger cable was inadequate also, primarily because the higher cable speeds used brought on disproportionately greater amounts of water. Thus, to accommodate the smaller, faster moving cable, more needed to be done than simply to decrease the diameter of the hole in the wiping elements to match the diameter of the smaller cable.